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Moto Mind

Moto Mind is a technical blog written by Paul Olesen who is a powertrain engineer working in the motorcycle industry. The blog covers a wide variety of topics relating to two and four stroke engine performance, design, and optimization.

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Premix Once - Measure Twice

Posted by
Paul Olesen
,
September 01, 2014
·
5,642 views

[color=rgb(0,0,0)]Premix Once - Measure Twice[/color]

[color=rgb(0,0,0)]I cringe when I see someone guess at the proper amount of oil to mix with their fuel when filling up their two-stroke dirt bike, snowmobile, jet-ski, or even weed whip. Manufacturer’s spend an awful lot of time figuring out what the right amount of oil is for a given engine application so when I see someone add a splash here and a splash there and call it good it worries me. If you’re one of those folks maybe after reading this it will worry you too.[/color]

[color=rgb(0,0,0)]Adding too little oil may lead to improper lubrication of the crank bearings, rings, piston, and rod bearings causing premature failure due to excessive wear and increased friction. You might think using less oil will save you a few dollars, will lead to more horsepower, or will keep your spark plug from fouling. Let me assure you that buying another quart or gallon of oil is much cheaper than having to replace an entire top and/or bottom end. Personally I have not come across a single study that proved less oil lead to more horsepower. I have ran oil mixtures as rich as 20:1 and have not had any problems with the bike fouling plugs. In my opinion, plug fouling occurs from poor combustion (possibly caused by combustion chamber shape, spark strength, or ignition timing) not the amount of oil in the mixture itself. [/color]

[color=rgb(0,0,0)]Let’s consider the effects of having an oil mixture that continually varies each time the bike is filled with fuel. As an example let’s say that the bike and carburetor is set up to run at a fuel/oil ratio of 40:1. What happens if we get generous with the amount of oil we add when we fill the bike up? Let’s say after we finish filling we end up with a fuel/oil mixture that is 20:1. Now the bike has much more oil in the fuel mixture than there was originally. There is no question that the engine will be well lubricated, but will the engine perform better or worse? Assuming that no changes are made to the carburetor to account for the richer oil mixture, the engine will most likely run worse. The reason being the amount of fuel able to pass through the orifice of the main jet, pilot jet, and needle circuit is reduced due to dilution caused by more oil. This will cause the bike to run lean and may lead to problems! While you may think you are doing the engine a favor by giving it more lubrication, unless the carburetor is adjusted to compensate for this change, you are actually increasing the chances of doing damage to the engine by running it lean. [/color]

[color=rgb(0,0,0)]On the flip side we could decide to take our engine that is set up to run a 40:1 fuel/oil mixture and use less oil. Let’s say we are down to the last quart of oil and need to get a couple bikes through a weekend of riding so we skimp and run the bikes at 80:1. In this case the opposite will happen. Since there is less oil in the fuel/oil mixture, more fuel will be able to flow through our carburetor circuits, thus causing the bike to run rich. A rich bike is much better than a lean bike, but what if there is no longer enough oil to adequately lubricate the engine? If there isn’t enough oil to lubricate the moving components within the engine, it is highly likely that engine components will wear faster, run hotter, and ultimately fail.[/color]

[color=rgb(0,0,0)]My advice to you would be to take the extra five minutes every time you mix to measure out the amount of gas and oil precisely. That way each time you fill the bike up you are giving your engine the most consistent fuel/oil mixture possible. Taking the time to do this will lead to more consistent performance, maintenance intervals, and save you a lot of money on an avoidable rebuild. [/color]

[color=rgb(0,0,0)]If you don’t already have an oil measuring container go out and pick one up for a couple bucks and throw it in with your riding supplies so you are never in the situation where you have to guess. Another tip I want to share with you is when you are at the gas station filling up your container with premium, let the first gallon of fuel go to your car or truck. By doing this you purge the gas pump’s hose of whatever blend was previously dispensed and ensure you are in fact getting premium for your toys. Once done filling and mixing, I like to label my gas jugs with the date I mixed them and with the fuel/oil mixture I mixed. Doing these simple things will help avoid confusion down the road and a keep your engine healthy.[/color]

Moto Mind - Empowering and Educating Riders from Garage to Trail[color=rgb(40,40,40)]If you'd like to follow my blog, click the "follow this blog" button in the upper right. I'd love to have you. [/color][color=rgb(40,40,40)] [/color]

Paul, what if you are on a long trail loop and have to refuel on the way. What do I carry, and what is the best way to refuel and maintain the proper ratio?

Good question Old Plonker. I'm going to assume this is a loop you've never done before and that you won't know how much fuel you'll need to add at your stop. I think the easiest thing to do would be to carry with you a mixing table which indicates quantities of oil for various amounts of fuel added. Please follow the link to a nice website with this information. You could recreate this table in a spreadsheet or print the one on the website and carry it with you. If you fuel up and end up between gallons you can use the table and interpolate the correct amount of oil to add. Whether you carry oil with you or buy it at the gas station a good oil bottle will always have a metering system on the side so you can determine how much you have added and how much is left. Just be sure to add slowly and accurately!

When adding fuel and oil at the pump I think the best way, albeit a little time consuming would be to add a set amount of fuel (let's say a gallon for simplicity sake) then the appropriate amount of oil until you've filled your tank. This way fresh fuel from the spigot is helping mix in the oil. Once you've topped off your tank you can add the remaining amount of oil. Put the cap on, give it a little shake to help mix the remaining amount of oil you just added, and you should be good to go. Once you start the bike up and ride off the vibration from the engine and terrain should do a nice job mixing the fuel and oil too.

Hope this help and be sure to follow my blog if you haven't already done so! Thanks!

I guess I should proof read my comments.I meant to say that when you put to much oil in your mix,your bike will run lean.If you don't put enough in the mix,your bike will run rich and foul your plugs.Its the gas that fouls plugs most of the time,not the oil.

Say for some reason that you don't ride for a while and you still have some mixed fuel in your jerry can, how long is to long before you shouldn't use that mixed fuel anymore?

That is a hard question for me to answer because there are so many variables. Fuel blends vary by manufacturer, from state to state, and by season. Things like temperature and humidity will also affect the life of gasoline as well as how/where the fuel is stored. I've seen gas that has lasted over a year and I've seen gas that has made it less than a month. Personally, for anything recreational I try to use the fuel up within two weeks of purchase, however this doesn't always happen and fuel that has sat for a month or more hasn't given me any problems. As far as storage goes I store my fuel at room temp, out of direct sunlight, and in a dry environment.

Anytime I'm skeptical of fuel quality after it has sat for awhile I down cycle it into applications which require less performance. Things like lawn mowers and other utility equipment are good candidates for fuel that may not be up to snuff for motorcycles.

I have a sx 85 (Ktm 2 stroke), and I'm trying to figure out how much oil and what kind would be best to use. I use amzoil saber (recommend for small engines,) and that say (even the ktm Dealer) its fine to run this stuff at a maximum of 100:1??? I was running it at 60:1 and it seemed to be running fine. Should i start using just interceptor at 40:1 again?

My Question is "is it safe to run this saber at 70/80:1 in my 85 2 stroke?"

I have a sx 85 (Ktm 2 stroke), and I'm trying to figure out how much oil and what kind would be best to use. I use amzoil saber (recommend for small engines,) and that say (even the ktm Dealer) its fine to run this stuff at a maximum of 100:1??? I was running it at 60:1 and it seemed to be running fine. Should i start using just interceptor at 40:1 again?

My Question is "is it safe to run this saber at 70/80:1 in my 85 2 stroke?"

Thanks for your help.

I would start with whatever the the service manual suggests for your engine and go from there. I don't see any reason to skimp on oil, the less you use the less margin you have and the more likely the engine will wear out faster. There have been no studies done, to my knowledge, that have indicated less oil has made more power, however the contrary is true. For what its worth I run a castor/synthetic blend at 30:1.